The rotor of an electrical machine may comprise a magnet secured to a shaft. As the rotor rotates, radial forces stress the magnet. Many magnets are relatively brittle and will fracture if subjected to excessive tensile stress. Consequently, a reinforcing sleeve may be provided around the magnet.
Conventional methods of securing a sleeve about a magnet include press-fitting the magnet into the sleeve. However, owing to tolerances in the outer surface of the magnet and the inner surface of the sleeve, there will be locations at which the magnet is not in direct contact with the sleeve. Stresses acting on the magnet will not therefore be transferred to the sleeve at these locations. As a result, the magnet may fracture. Although the sleeve may prevent the magnet from breaking apart, the fracture may bring about a slight mass redistribution of the magnet, resulting in rotor imbalance.
An alternative method involves applying a high-viscosity adhesive to the inner surface of the sleeve and the outer surface of the magnet. The sleeve is then slid over the magnet and the adhesive is cured. However, there are several disadvantages with this method of securement. First, owing to tolerances and other imperfections in the surfaces of the magnet and the sleeve, there may be locations at which the adhesive does not extend fully between the magnet and the sleeve. Consequently, there will be locations at which stresses are not transferred from the magnet to the sleeve, which may result in magnet fracture. Second, the sleeve is poorly constrained relative to the shaft. As a result, the position of the sleeve may drift with use of the rotor due to adhesive creep. Any movement in the sleeve will introduce a rotor imbalance. Third, tolerances in the magnet and the sleeve may mean that an excessive amount of adhesive must be applied to the surfaces in order to achieve a good adhesive join. Consequently, the method is particularly wasteful in terms of adhesive. Fourth, owing to the relatively high viscosity of the adhesive, cleaning excess adhesive from the rotor is generally difficult.